Valve train devices that have a mechanism for changing the lift of gas exchange valves are already known. The invention is based on a valve train, in which the gas exchange valves, in particular inlet valves and outlet valves of an internal combustion engine, are directly or indirectly actuated by means of a camshaft. Cams that are adjacent on the camshaft and have different cam shapes, which cams are combined to form a cam unit, are provided to change the valve lift. By means of an axial movement of the cam units on the camshaft, one cam of the cam group in each case is made to engage with the corresponding gas exchange valve and the lift of the gas exchange valves is produced in accordance with the cam shape. A push rod, which runs parallel to the camshaft, with corresponding elements for engagement on the cam units, is provided to move the cam units.
A valve train for gas exchange valves of an internal combustion engine with a movable cam carrier having a continuous groove running over the periphery emerges from the published application DE 10 2007 061 353 A1. The cam carrier is non-rotatably but axially movably mounted on a camshaft and has a plurality of axially spaced-apart cams with different cam elevations, which are combined to form a cam group for the respective gas exchange valve. An engagement element, which carries out a lift in the axial direction according to the course of the continuous groove, engages continuously in the continuous groove. The engagement element can be locked in specific positions, so the cam carrier is forced to carry out an axial movement to thus change over between the cams to drive a gas exchange valve.
However, the system described requires a great deal of installation space, so it is only possible to use it in internal combustion engines with adequate cylinder spacing. Furthermore, an actuator is necessary to lock the engagement element for each movable cam carrier, said actuator having to be activated at the correct time.
The published application DE 10 2009 030 373 A1 describes a valve train for an internal combustion engine with a variable lift gas exchange valve actuation. The cam shaft of the valve train comprises a carrier shaft and a cam piece, which is non-rotatably but axially movable thereon. The cam piece contains cam groups of directly adjacent cams with different elevations. A gate is provided at the end of the cam piece in order to axially move the cam piece in relation to the carrier shaft by coupling in an actuating element. The cam piece revolving with the carrier shaft is mounted in a camshaft bearing.
The adjusting gate on the cam piece is arranged outside the cam shaft bearing, so additional axial installation space is produced for each cam piece. A separate actuating element is required for each adjustment direction. As a result, two actuators are used for each cylinder of the internal combustion engine. This increased control outlay and component outlay in conjunction with the space requirement is a main drawback of the device.
A valve train with variable lift gas exchange valve actuation emerges from the published application D10 2010 013 216 A1. The valve train consists of a camshaft, which is constructed from a carrier shaft and cam pieces mounted non-rotatably but axially movably thereon. A cam group of directly adjacent cams with different elevations and an axial gate is located on the cam piece. The cam piece is also provided with a bearing journal and mounted thereby in a cam shaft bearing. The bearing journal on the adjacent axial gate are overlapped by the camshaft bearing, so the actuating element is positioned on the camshaft bearing to move the cam piece and runs radially through the bearing point to engage in the axial gate.
As the axial gate and also the bearing journal are overlapped by the camshaft bearing, the bearing point has to be particularly wide to provide the required bearing face in accordance with the bearing loads. This leads to an increased installation space requirement. The system can only be used in internal combustion engines with adequately large valve spacing or cylinder spacing. Moreover, an actuating element is also required in each case in this system for each cam piece. This stands in the way of the current development aim of compact internal combustion engines.
A changeable valve train of an internal combustion engine is described in the published application DE 10 2007 022 145 A1. The camshaft of the valve train comprises a drive shaft with at least one cam piece non-rotatably but axially movably arranged thereon with a group of axially adjacent cams. The cams have different cam elevations here. The cam piece is radially supported together with the drive shaft and with an additional bearing bush coaxially mounted on the cam piece in a cam shaft bearing point. The additional bearing bush is axially movable together with the cam piece in relation to the camshaft bearing point and the drive shaft, the bearing bush being secured against rotation in the camshaft bearing point. The cam piece and the bearing bush are thus accordingly arranged coaxially one above the other on the drive shaft and mounted in the camshaft bearing point. A pin is positioned in the bearing bush parallel to the rotational axis of the drive shaft, said pin carrying out an oscillating movement by means of a guide introduced in the revolving cam piece with an axial lift. To initiate a switching process, the pin is briefly axially fixed by a fixing mechanism at the corresponding point. As a result, the cam piece is supported on the fixed pin. The cam piece is forced to carry out an axial movement and a switching process is initiated.
The complex control and the component outlay connected therewith are to be mentioned as the drawback of this device for changing the lift. A separate fixing mechanism, which has to initiate a switching process at the right time, has to be provided for each changeover device of a cam piece. The pin moreover leads to high loads in the guide groove during the switching process. Furthermore, a high degree of wear is to be expected on the pin as the pin has to be clamped for each switching process.
U.S. Pat. No. 5,129,407 A describes an adjusting device for a camshaft, with which the camshaft can be axially moved. Owing to the movement of the camshaft, different cams with different cam elevations can be positioned to actuate the gas exchange valves. An axial gate, with which an axial movement in the two directions is made possible, is provided to move the camshaft. For this purpose, the gate has two contours running mirror-symmetrically over the periphery, each of the two contours producing the lift required for the adjustment to adjust the camshaft. Provided between the two contours is a region, in which the adjusting pin that is permanently engaged can be positioned without contact with one of the two peripheral contours. To initiate a switching process, the adjusting pin is moved in the axial direction and fixed in order to come into contact with one of the two contours and to initiate the axial adjustment of the camshaft in the corresponding direction.
The camshafts and the cams positioned thereon of modern internal combustion engines with a plurality of cylinders do not, however, allow a simultaneous adjustment of the cams nor, therefore, a movement of the entire camshaft as no overlapping of the cam base circles is produced between the various cylinders. Moreover, the gate causes an enlargement of the overall length of the camshaft.
A valve train for gas exchange valves of an internal combustion engine is known from the published application DE 10 2009 039 733 A1, in which a cam unit with a plurality of different can tracks arranged next to one another is non-rotatably and axially movably mounted on a camshaft and the cam unit can be axially moved by an adjusting means. The adjusting means comprises an adjusting element movably mounted within the camshaft to transmit the adjusting movement to the cam unit. For this purpose, the adjusting element has, on its periphery, a running track profile, which cooperates with a coupling element, the coupling element furthermore being connected by a recess in the camshaft to the cam unit. An axial movement of the cam unit on the camshaft is brought about by a movement of the adjusting element relative to the camshaft. It is described as a configuration variant that the adjusting element is rotatably mounted in the camshaft and that the running track profile substantially extends in the peripheral direction on the adjusting element. The change between two cam tracks on the cam unit to actuate the gas exchange valves is achieved by an axial offset in the running track profile, which is produced by a relative rotation of the adjusting element in relation to the camshaft.